Superheterodyne receiver



Dec. 13, 1938.

W. VAN B. ROBERTS SPERHETERODYNE RECEIVER' Filed June lO, 1937 m Y v TML INVENTOR m E l m m 5. N NOR O V T R .T m

Patented Dec. 13, 1938 UNITED STATES anche PATENT OFFICE SUPERHETERODYNE RECEIVER of Delaware Application June 10, 1937, Serial No. 147,402

Claims.

My present invention relates to superheterodyne receivers, and more particularly to superheterodyne receivers capable of short wave reception without image interference.

The main object of my invention is to provide a superheterodyne receiver system for receiving signals free of image interference where the separation between carrier frequencies of adjacent signals is relatively large.

l The central feature of the present method is the choice of an intermediate frequency so low that no image interference is produced by signals in the channel adjacent to that of the desired signal. Since this may necessitate employing an intermediate frequency so low that its side bands will include frequencies lying in a range of modulation frequencies of the desired signal, a balanced converter circuit is provided to prevent currents of modulation frequencies, produced by ordinary detector action in the converter, from entering the intermediate frequency filter.l The balanced converter arrangement bucks out all ordinary detection effects while yielding the cumulative effect of the beats between incoming signals and the local oscillations since the latter are differentially applied to the two converter tubes.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method -of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 graphically illustrates the operation of the present receiver,

Fig, 2 schematically shows a superheterodyne receiver embodying the invention.

Referring to the drawing, and specifically to v Fig. l, let the line S represent the carrier frequency of the desired station, its channel width being B. If, now, the heterodyne frequency H is chosen to differ from S by more than and if the frequency separation between H and F-H H-S), then an intermediate frequency (Cl. Z50-20) band pass filter will be able to reject all beat frequencies between H and the adjacent channel A, while passing the beats between H and at least one side band of S and the carrier S of the desired station.

In Fig. 2 is shown a receiver circuit embodying the invention. The signals collected at A, which may be a grounded antenna, are impressed on the tunable signal input circuit I of the converter, or combined first detector-local oscillator. 10 The latter comprises tubes 2 and 3; each being of the pentagrid converter stage with electron coupling between signal and oscillator networks. The oscillator grids 4 and 5 are connected to opposite sides of coil E; the mid-point of the 15 latter isvconnected to the common cathode lead through leak resistor 1 by-passed by condenser B. The signal grid bias resistor 9 connects the common cathode lead to ground. The tunable tank circuit Ill is magnetically coupled to coil 6; the oscillator anode electrodes II and I2 are connected to opposite sides of coil I3, and the latter is magnetically coupled with tank circuit I0 and coil 6.

The signal grids I4 and I5, each shielded by positive screen grids, are connected to the high potential side of selector circuit I. The rotors of the tuning condensers of circuits I and I0 may be uni-controlled in adjustment. The plates I6 and I'I are connected to opposite sides of the primary I8 of the transformer M1; the primary being tapped to energize the plates. Proper filter networks IIJ-I9 are inserted in the leads to plates I6 and I'I respectively. The numeral 20 designates the I. F. band pass lter network, which includes one or more amplifiers.

The converter tubes 2 and 3 provide a balanced converter network; the local oscillations are differentially applied to grids 4 and 5, while the signals are applied in parallel to the signal grids I4 and I5. The plates I6 and II are arranged in bucking relation. In this way all ordinary detection effects are balanced out in the common plate circuit of the converter; the cumulative effect of the beats between incoming signals and local oscillations will be secured in the common plate circuit. Thus, modulation frequencies, produced by detection in the converter, are prevented from entering the network 20. 50

The network 20 is conventionally represented, since it is thought that those skilled in the art will readily be able to construct the filter and amplifier necessary to carry out the required transmission as Sci ,forth herein. Generally 55 Cil speaking, and referring again to Fig. 1, the network 20 is designed to reject all beat frequencies between H, the oscillation frequency, and the adjacent channel A; it passes the beats between H and at least one side band of S and the desired carrier S. The range of intermediate frequency currents in such a case may include frequencies within the range of modulation frequencies of the desired signal, as explained before, and, to prevent these frequencies from passing through the second detector, a balanced second detector arrangement, well known in itself, is used. The output of this second detector is composed of the products of detection, and does not contain any of the frequencies impressed upon it,

The second detector may use a tube 2l of the duplex diode triode, or pentode, type; the diode anodes 22 and 23 are connected to opposite ends of secondary coil 24 of coupling transformer M2. The diode load resistor 25 is connectedy from the midpoint of coil 24 to the grounded cathode 28'; the audio component of detectedl signal current is impressed on negatively biased control grid 26. The plate 21 is coupled to any desired audio network.

If the intermediate frequency side` bands include currents having frequencies less than onehalf of the highest modulation frequencies, the second detector will by its frequency doubling action convert these frequencies to harmonics which still fall within the desired range of modulation frequencies. To avoid distortion by such harmonics it is preferable to add a further limitation on the choice of the intermediate frequency. Ihat is, the minimumy frequency of the intermediate frequencyl channel should be chosen at least one-half the highest modulation frequency. That is,

This equation together with, the essential equation given previously, limits the choice of intermediate frequency to a range of values within which proper operation of the invention is possible. Within this range of possible values it is, of course, preferable to choose the intermediate frequency as far as possible from the limitations imposed by the equations so thatV the I. F. filter network 2G will not be required to cut off at its limits any more sharply than necessary.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art thatv my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. A superheterodyne receiver arrangement, free from image interference, characterized by relatively wide separation between adjacent signal channels, of the order of 20 kilocycles, in the range of frequencies to be received, and an intermediate frequency transmission network whose transmission characteristic is chosen to make the image frequencies of a desired signal lie wholly within the clear space between the desired signal and an adjacent channel.'

2. A superheterodyne receiver arrangement, free from image interference, characterized by relatively wide separation between adjacent signal channels in the range of frequencies to be received, and an' intermediate frequency transmission network whose transmission characteristic is chosen to make the image frequencies of a desired signal lie wholly within the clear space between the desired signal and an adjacent channel, and a balanced converter network producing intermediate frequency energy.

3. AV superheterodyne receiver arrangement, free from image interference, characterized by relatively wide separation between adjacent signal channels in the range of frequencies to be received, an intermediate frequency transmission network whose transmission characteristic is chosen to make the image frequencies of a desired signal lie wholly within the clear space between the desired signal and an adjacent channel, and a balanced detector for demodulating the intermediate frequency energy.

4. A superheterodyne receiver arrangement, free from image interference, characterized by relatively wide separation between adjacent signal channels in the range of frequencies to be received, and an intermediate frequency transmission network whose transmission characteristic is chosen to make the image frequencies of a desired signal lie wholly within the clear space between the desired signal and an adjacent channel,V and the intermediate frequency channel including frequencies within the audible range.

5. In a superheterodyne receiver,'a balanced converter network, a balanced demodulator network, and an intermediate frequency network coupling said networks, said coupling network having its constants chosen so that it rejects all beat frequencies between the oscillator frequency and an adjacent image frequency while it transmits all beat frequencies between the oscillator frequency and at least one side band of the carrier of a desired station and the said carrier.

WALTER VAN B. ROBERTS. 

